Question 104530
I don't think you can predict whether it'll turn into a fraction until you're working through it. Let's start. 
1.{{{3x - 6y = 9}}}
2.{{{x -2y = 3}}}
The easiest way to solve is to use equation 2 and solve for x. Then substitute your value for x into equation 1. 
2. {{{x-2y=3}}}
{{{x-2y+2y=3+2y}}} Additive inverse of 2y.
{{{x=3+2y}}} 
Now use your result from equation 2 and plug it into the value for x in equation 1.
1. {{{3x-6y=9}}}
{{{3(3+2y)-6y=9}}} Substitute. 
{{{9 + 6y - 6y = 9}}}
{{{9=9}}}
Huh? What happened? 
The problem you've run into is the equations are dependent, which means they are the same equation in disguise.
You don't have enough information to solve the equations. 
In fact, if you multiply the second equation by 3, you get the first equation. 
{{{3(x-2y)=3*3}}}
{{{3x-6y=9}}}
The equations are dependent, there is no solution.